1. Field of the Invention
The present invention relates to an article, such as an aircraft wing, having an aerofoil section with a variable geometry expansion surface for the control of shock strength.
The invention is particularly applicable to wings for transport aircraft where it is intended to combat transonic wave drag, to which such wings are susceptible when the aircraft is flown in off-design conditions. However, the invention may also find use in the wings of other aircraft types as well as possibly for control surfaces rather than wings.
2. DISCUSSION OF PRIOR ART
It is well known that air-flows over aircraft wings at high subsonic free stream Mach numbers exhibit shock waves due to localised excursions into supersonic flow. The supercritical wing forms of modern transport aircraft ensure that the effects of such shock waves are minimised at the design cruise Mach number, altitude and coefficient of lift. However, it is recognised that these shock waves can increase significantly in strength with small departures from the design conditions, such as a change in the angle of incidence or an increase in Mach number. These shock waves create drag, which is known as transonic wave drag, and can lead to the breakdown of ordered flow. Operation under these conditions is obviously undesirable since it increases aircraft fuel consumption. Moreover, the flow breakdown may cause problems of aircraft control or reduction in aircraft structural life through buffet loading.
In other, non-supercritical, wing sections designed for natural laminar flow, the aerofoil section necessary to maintain the laminar boundary layer as far aft as possible (for the benefit in drag reduction that this conveys) produces transonic wave drag which unfortunately increases rapidly in strength as either Mach number or lift coefficient rise above the design condition value. This places a severe limitation on the operating band of Mach numbers and lift coefficients for laminar flow aircraft.
Various solutions have been proposed to alleviate the problem of transonic wave drag associated with aircraft wings. One approach has been to incorporate perforations or slots in the expansion surface of the wing at both the upstream and downstream sides of the shock location, with these being linked through a plenum chamber inside the wing. This arrangement makes use of the pressure differential that exists in the airflow adjacent the wing. Air is bled from the higher pressure region downstream of the shock and reintroduced in jets upstream of the shock. These jets create a ramp to oncoming flow and thereby cause compression waves which weaken the shock. One of the drawbacks of this solution is that a boundary layer drag penalty is imposed across the entire range of flight conditions, due to the presence of these holes or slots. This drag penalty may outweigh the advantage gained by reduction of transonic wave drag in terms of overall operating costs.
Another approach to reduction of transonic wave drag and associated buffeting is to configure the aerofoil section so as to produce the same pre-shock compression achieved by the method described above. For example, at page 673 of the Journal of Aircraft Volume 25, No 8, dated August 1988, Tai and co-workers disclose an aerofoil with a bulge on the expansion surface at a position approximating to that of the shock wave.
However all aerofoil sections represent a compromise between various conflicting requirements and it is unlikely that any specific configuration optimised with this specific end in view would have a beneficial or neutral effect across the whole range of flight conditions. It is more likely to be the case that, in securing a reduction in the off-design point wave drag, some increase in viscous or wave drag is caused at another condition. Accordingly it is unlikely that a wing of this nature would be of such advantage as to extend the flight envelope whilst reducing overall fuel consumption.